Because it is based on random mutations in DNA. Over time, a mutation may occur that gives a species an advantage, or an ability to cope with changing conditions. But this new advantage must be passed to many generations of offspring in order for the whole species to benefit. This process takes time and luck. For example, the mutation that allows adult humans to digest milk (to continue producing the enzyme lactase that babies produce), took around 7,000 years to become prevalent in Northern Europe. Learned Science would probably say that polar bears don’t have 7,000 years to 1) get lucky with a mutation and 2) spread that mutation throughout their population.

There’s still a long shot possibility that changes in Polar Bears’ environment (warmer temps) will trigger a genotype-environment interaction in which long-dormant genes will be activated by the challenges of new environmental conditions, thereby allowing them to adapt more quickly. Following copied from an article, google the term to get more: